The growing role of wavelength division multiplexing (WDM) in optical communication systems has driven the need for semiconductor laser sources that can emit at different wavelengths. Current WDM systems for example use over 100 wavelengths spanning a range from about 1525 nm to about 1600 nm. With the advent of Raman amplifiers and new high bandwidth filters, WDM transmission wavelengths are expected to encompass the so called S-band nearing 1300 nm. Lasers based on InGaAsN can potentially cover the entire wavelength range from 1200 to 1600 nm.
Fabricating a number of semiconductor lasers spanning a wavelength range has proven to be a difficult challenge. One method is to adjust the composition of the active material by changing the growth recipe of each laser device. In particular, the active layer of each laser may differ according to the desired output frequency. The variation may be achieved by varying the epitaxial growth of each active layer as described in U.S. Pat. No. 6,167,074 entitled “Monolithic Independently Addressable Red/IR Side by Side Laser” by Sun et al. filed Feb. 24, 2000 and hereby incorporated by reference.
One problem with a multiple recipe approach is the difficulty of implementation in manufacturing settings. In particular, applying different recipes can result in delays and reproducibility problems associated with recipe changes. Multiple recipes also increase costs. For example, different reaction chambers may typically be used to accommodate different recipes. Some implementation would involve a regrowth. Etching and regrowth processes are undesirable because of the high cost associated with pre-regrowth sample preparation, the epitaxial regrowth process itself, and the manufacturing logistics involved. An additional drawback is the non-planar morphology that results.
Alternative approaches to produce laser arrays with varying emission wavelength are based on growth techniques such as migration-enhanced epitaxy and temperature-graded substrate condition. However these techniques are complicated, time consuming and are difficult to precisely control. Failure to maintain tight controls results in difficulty controlling the emission wavelength.
Thus a simpler and less expensive way of tuning the wavelength output of an InGaAsN laser is needed.